(1) Field of the Invention
The invention relates to a transmission system for transmitting binary data symbols in synchronous consecutive symbol period intervals having a duration of T seconds, and more specifically to a transmission system in which each data symbol is converted into a bi-phase symbol whose waveform in an interval T is given by one cycle of a carrier having a frequency 1/T or a carrier whose phase is shifted through 180.degree. relative thereto, depending on the modulating binary data signal.
The power spectrum of such a bi-phase modulated signal has a zero at the frequencies 0 Hz and 2/T and a maximum between 0 Hz and 1/THz in the region of the frequency 1/T. Bi-phase modulation is used inter alia in those cases in which the transmission system, the physical transmission medium included, is not suitable, or is poorly suitable, for the transmission of the d.c. components and the low frequencies of the signal. The bi-phase modulated signal contains little energy at the low frequencies. For which reason a high-grade detection of the bi-phase signal is possible in the absence of these frequencies in the transmitted signal.
In addition the bi-phase modulated signal comprises sufficient clock information to guarantee simple generation of a clock signal at the receiving end.
(2) Description of the Prior Art
Transmission systems for transmitting binary data symbols in which the data symbols are transmitted in non-modulated form, for example in the form commonly referred to as "non-return-to-zero" (NRZ) symbols, make efficient use of the available bandwidth. The power spectrum of such an NRZ-signal has its maximum at zero frequency and a zero at the frequency 1/T. The frequency 1/T corresponds to the symbol frequency of the data signal to be transmitted and is usually designated the bit rate.
As is known, a (theoretical) bandwidth of 1/(2T) is sufficient for the undistorted transmission of NRZ-signals. Generally, the receive filter of such a system will be in the form of a Nyquist-1 filter having a cut-off frequency equal to half the bit rate and having a frequency response which extends to the bit rate and is zero thereabove.
Known disadvantages of the use of NRZ-signals in data transmission systems are on the one hand the absence of adequate clock information to enable simple and reliable clock generation at the receiving end and on the other hand the presence of a considerable amount of low-frequency information in the transmitted signal. As a result thereof, in those cases in which the transmission system, the physical transmission medium included, is not suitable, or is poorly suitable, for the transmission of these low frequencies, for example because of the presence of transformers or high-pass filters, a serious distortion of the received pulses will occur, which significantly interferes with a reliable detection of the signals.
Bi-phase modulation is one of the known methods of transmitting binary data signals without the above-described disadvantages of the presence of a d.c. component and low-frequencies, and with the advantage of sufficient clock information. In principle bi-phase modulation is suitable for use in transmission systems in which suppression of the low frequencies in the signal to be transmitted occurs. However, bi-phase modulation doubles the signalling rate relative to the original data signal, which is accompanied by an increase of the required frequency band. With bi-phase modulation this frequency band extends in principle from 0 Hz to frequency 2/T so that generally for bi-phase modulated signals the receive filter will be formed by a Nyquist-1 filter having a frequency response which extends to twice the bit rate and which is zero thereabove.
The transfer function F(1) of a Nyquist-1 filter for bi-phase modulated signals is, for example, given by: ##EQU1## where .omega.=2.pi.f is the angular frequency.
Such a Nyquist-1 filter is optimal as regards the detection of the signals without intersymbol interference, but is less optimal as regards the signal-to-noise ratio. Thus, the above filter has an equivalent noise bandwidth which is twice the noise bandwidth of the customary receive filter having a Nyquist-1 characteristic for non-modulated NRZ-symbols. In the case of white noise at the input, this results in a 3 db loss in signal-to-noise ratio compared with NRZ-transmission.